The First Heretic details the fall to Chaos of the PrimarchLorgar and his Word Bearers, the XVII Astartes Legion. The story is told largely from the point of view of Argel Tal, a Captain of the Word Bearers, and spans about 50 Terran years, starting 47 years before the events on Istvaan V and leading up to the time when the Word Bearers are on their way to assault Calth as related in Book 8. Following the humiliating censure of Lorgar and the XVII Legion by the Emperor himself for introducing religious worship of the Emperor as the God of Mankind on the world of Khur, Lorgar is swayed by two of his most trusted lieutenants Kor Phaeron and the the Chaplain Erebus, who were already in secret allegiance with Chaos. Eventually both Primarch and his Legion abandon the Imperium following the Pilgrimage of Lorgar into the Eye of Terror, and in full accord with Horus take part in the Istvaan V Drop Site Massacre.